LCD screens for such display systems include passive LCD screens and Thin Film Transistor (TFT) LCD screens.
A passive LCD panel includes two orthogonal arrays of parallel conductive tracks in the form of rows and columns. A layer of liquid crystal material is placed between the two arrays thereby forming a capacitor at each intersection of the orthogonal arrays. The capacitor of an intersection is charged by placing a voltage across the corresponding conductive tracks. When the capacitor is charged, a light path is produced through the liquid crystal material at the intersection thereby generating a pixel cell.
In a TFT LCD, the liquid crystal material is placed between a planar electrode and an array of separate pixel electrodes. Each pixel electrode is coupled to the drain of a transistor switch. The transistor switch is located at the intersection of two orthogonal conductive tracks (row and column tracks). The source of the transistor is coupled to the column track and the gate is coupled to the row track. The transistor switch turns on when a voltage is applied on the row track. In response to the transistor turning on, the capacitor formed between the pixel electrode and the planar electrode charges up to a data voltage applied to the column track. When the transistor is subsequently turned off, the charge stored in the capacitor remains. A light path is produced through the liquid crystal material at the pixel electrode thereby generating the pixel cell.
Passive and TFT LCD screens can exhibit a brightness non-uniformity when the displayed image is generated by a grey scale video signal. The non-uniformity error takes the objectionable visual appearance of spurious brightness variations distributed across the LCD panel. These variations limit the quantity of grey scale brightness levels that can unambiguously be generated.
The brightness non-uniformity can arise from variations in thickness of the liquid crystal layer. This effect is particularly significant where the liquid crystal layer is made thin (typically 4 um) to reduce the transient response period of the LCD panel. In reduced layer thickness LCD screens, any slight variation in the layer thickness causes a corresponding variation in brightness. In colour LCDs, further thickness variations can be introduced by colour filter layers. These further variations add to the effect.
The brightness non-uniformity can also arise from variations in molecular orientation of any liquid crystal alignment layers applied to inner surfaces of the LCD panel.
Furthermore, the brightness non-uniformity can arise from variations in electrical characteristics of the row tracks, the column tracks or the thin film transistors (of a TFT LCD), or any combination thereof.